1. Field of the Art
The present invention relates to a process for producing ethylene copolymers. More particularly, it relates to a process for producing ethylene copolymers by copolymerizing ethylene with an .alpha.-olefin having 3 to 10 carbon atoms characterized in that a polymerization catalyst having been prepared under a specific condition is used.
2. Prior Art
Recently, a catalyst consisting essentially of bis(cyclopentadienyl)zirconium chloride and methylalumoxane has been found to have excellent properties that it affords an extremely high polymerization activity in the copolymerization of ethylene with hexene and has a high sensitivity to hydrogen or comonomers to give a copolymer which has a Q value which is a ratio of M.sub.w /M.sub.n of 3 or less and thus has an excellent randomness, and the spotlight of attention has been focused upon it [Studies in Surface Science & Catalysis, "Catalytic Polymerization of Olefins" pp. 293-304, Kodansha/Elsevier (1986), W. Kaminsky: Preparation of Special Polyolefins from Soluble Zirconium Compounds with Aluminoxane as Cocatalyst].
This catalyst system is a homogeneous catalyst which is soluble in a solvent for polymerization and has no catalyst particles which are to serve as "cores" of polymer particles. Thus, when it is used for gas phase polymerization which is substantially performed under the formation of polymer particles, particle-forming reaction does not proceed uniformly and it may be impossible to operate the reaction system for a long period because of the troubles such as the generation of blocks of polymer due to the aggregation of the polymer particles or adhesion of the polymer to the wall of a reaction vessel or reactor.
For the purpose of solving the problem, there has been proposed a process in which the catalyst components are carried on a support of an inorganic oxide such as SiO.sub.2 or the like (Unexamined Published Japanese Patent Application Nos. 108610/1986 and 296008/1986).
However, in this process a catalyst which carries both the zirconium compound and the alumoxane is used and no further addition is made of an alumoxane which enhances the zirconium compound, and accordingly the catalyst activity per support remains at low levels. According to our knowledge, the method could also have a problem in view of its application for the gas phase polymerization method that there is an induction period before the generation of the activity and may thus want further improvement. In a gas phase polymerization method wherein polymer particles are maintained in a fluidized bed housed in a reaction vessel by a reaction gas flowing upwardly, the reaction vessel is ordinarily provided with a circulating system through which a gas phase in the reaction vessel is taken out from the reaction vessel and returned to the reaction vessel after it has undergone the temperature control. However, when a solid catalyst component introduced into the reaction vessel has a long induction period until the initiation of polymerization and thus will not rapidly develop its activity, the catalyst component will be carried away by the gas into the circulating system and polymerization proceeds in the circulating system and blocks it, so that it would be quite impossible to continue stable operation for a long period. Thus, the presence of the induction period in the gas phase polymerization is a fatal defect for its industrial application.
A method that an alumoxane is separately added to a solid catalyst component in which both a zirconium compound and an alumoxane are carried on the support to conduct polymerization is disclosed in Unexamined Published Japanese Patent Application No. 51407/1988. Methods that an alumoxane is separately added to a variety of solid catalyst components carrying a zirconium compound are disclosed in Unexamined Published Japanese Patent Application Nos. 22804/1988, 51405/1988, 54403/1988, 51489/1988, 61010/1988, 66206/1988 and 89505/1988 and the International Publication of WO 88/01626.
However, in such specifications, nothing may be described about the contact of the solid catalyst component and the alumoxane in a solution state just before they are fed into the reactor nor the concentration of the alumoxane on the contact of them. In Example 24 of the International Publication, particulate NaCl is used as a dispersant for gas phase polymerization.